1. Field of the Invention
The present invention relates to a tape driving apparatus for controlling the transportation of a lengthy tape such as a magnetic tape by a supply reel and a take-up reel, and more particularly to a tape driving apparatus which can make a tape travel control at a rapid response speed without causing tape damage.
2. Description of Related Art
In recent years, tape driving apparatuses have remarkably been popularized and are used in various ways by various people. In such circumstances, tape driving apparatuses are required which have a high-speed operating ability as basic performance and are easy to use and which do not damage magnetic tapes or the like as recording media.
One of conventional tape driving apparatuses satisfying the above requirements has been proposed by JP-A-57-169956. In the following, explanation will be made of a VTR as a typical example.
In the conventional tape driving apparatus, high-speed start stop is realized with no tape damage by supplying a predetermined motor torque to each of supply and take-up reels. For this purpose, the tape radius on each reel, the tape acceleration and the tape tension on each of the tape supply and take-up sides are detected by means of a plurality of detectors to determine the moment of inertia of each reel and to control the reel motors. The optimum motor torque to each of the supply and take-up reels is determined on the basis of the determined moment of inertia of each reel and an output of each detector. This principle will now be explained.
The moments I.sub.S and I.sub.T of inertia of the supply and take-up reels are determined using the following equations: ##EQU1## where .sub..gamma.S and .sub..gamma.T are the tape radii on the supply and take-up reels, .alpha. is the tape acceleration, T.sub.SO and T.sub.TO are the initial torques generated by the supply and take-up reel motors, and F.sub.S and F.sub.T are the tape tensions on the tape supply and take-up sides. Here and elsewhere in the specification, unless otherwise noted, the convention is observed that for torque T.sub.S or T.sub.SO generated by the supply reel motor and angular velocity .omega..sub.S of the supply reel, the positive direction is that of left-handed revolution, while for torque T.sub.T or T.sub.TO generated by the take-up reel motor and angular velocity .omega..sub.T of the take-up reel, the positive direction is that of right-handed rotation.
Next, using the moments of inertia I.sub.S and I.sub.T, the motor torques T.sub.S and T.sub.T generated by each of the supply and take-up reel motors are set as follows: ##EQU2## where .omega..sub.S and .omega..sub.T are the angular velocities of the supply and take-up reels.
In this manner, stable start/stop can be realized without damaging the magnetic tape or the like.
However, the above proposed construction involves the following problems. Namely, since tension sensors are required on the tape supply and take-up sides respectively in order to detect the moments of inertia, available mechanisms are restricted and the proposed construction cannot be employed in VTR's for citizens' use for which a small size is required. Also, since the motor torque for each reel changes depending on the output of the tension sensor, the control of the motor becomes unstable when a change occurs at a higher speed than the response speed of the tension sensor. Further, since the measurement of the moment of inertia of each reel is necessary upon acceleration and deceleration, the actual acceleration/deceleration operation has a time lag relative to a command. Furthermore, as the moments of inertia of both the reels and the motor torques are determined through an arithmetic process, the amount of operation required becomes large and hence a plurality of processors or an extremely expensive and high-speed processor must be used.